The present invention relates to a method of and an apparatus for wire electric discharge machining in which an electric discharge is generated in a gap between a wire electrode and a machining subject to machine the machining subject.
The electric discharge machining has become an indispensable technique as a machining technique for metal molds, etc., and has been widely used in the field of the metal mold machining in the automobile industry, electric appliance industry, semiconductor industry, and other industries.
FIG. 6 is an explanatory drawing that shows the mechanism of the electric discharge machining, and in this Figure, reference numeral 1 is an electrode, 2 is a machining subject, 3 is an arc column, 4 is a dielectric fluid, and 5 is machining dusts generated by an electric discharge machining process. The machining subject 2 is machined while the following cycle of processes (a) to (e) (corresponding to (a) to (e) in FIG. 6) are repeated. Specifically, these processes include (a) a formation of an arc column 3 by the generation of a discharge, (b) a local fusing process and evaporation of the dielectric fluid due to the thermal energy from the discharge, (c) generation of evaporation explosion force of the dielectric fluid 4, (d) scattering of fused portions (machining dusts 5) and (e) cooling process, solidifying process and insulation-recovering process between electrodes through 9 the dielectric fluid.
Among electric discharge machining processes, this invention particularly relates to boring process, cutting process and the like. With respect to the wire electric discharge machining, there have been strong demands for high precision, and, for example, in the machining process for high-precision metal molds used in the semiconductor industry, etc., there has been a demand for a high machining precision of approximately 1 to 2 xcexcm.
FIG. 7 includes drawings for explaining an example of the machining processes involved in the conventional wire discharge machining operation. In FIG. 7, reference numeral 1a is a wire electrode, 2 is a machining subject, 4a is water, which is a dielectric fluid, 6 is an initial hole. FIG. 7(a) represents a first cut that corresponds to a coarse machining process, FIG. 7(b) represents a second cut that corresponds to an intermediate finish machining process after the coarse machining process, and FIG. 7(c) represents a third cut that corresponds to a final finish machining process.
In the first cut machining example of FIG. 7(a), the wire electrode 1a is inserted through the initial hole 6 so as to carry out a boring process through the machining subject 2. In the case of the first cut of this type, strict surface roughness and precision are not required since the surface roughness and precision are finished in the succeeding processes, and in particular, it is essential to increase the processing speed so as to improve the productivity. In order to increase the processing speed in the wire electric discharge machining operation, water 4a is strongly discharged to the gap between electrodes so as to effectively remove the machining dusts from the gap between electrodes. Moreover, in order to eliminate irregularity in discharged water 4a to the gap between electrodes and also to prevent disconnection of the wire electrode 1a, a method is adopted in which a processing vessel not shown is filled with water 4a in which the machining subject 2 is immersed.
The processes of the second cut ((b) in FIG. 7) and the third cut ((c) in FIG. 7) after the first cut ((a) in FIG. 7) are also carried out in water 4a that is a dielectric fluid.
FIG. 8 shows one example of a voltage and a current waveform of the gap between electrodes. In FIG. 8, V represents a voltage between electrodes, I represents a current between electrodes and represents time. A timing T1, a voltage is applied between the wire electrode 1a and the machining subject 2. When the voltage is applied, an attracting electrostatic force is exerted between the positive electrode and the negative electrode so that the wire electrode 1a having a smaller rigidity is pulled. toward the machining subject 2 by the electrostatic force. The wire electrode 1a starts vibrating because of such attraction, and it becomes very difficult to carrying out machining process at high precision because of the vibrating electrode.
At timing T2, the discharging energy generates a vaporization explosion force in the dielectric fluid (for example, see (c) in FIG. 6), a greater force is exerted on the wire electrode 1a by the vaporization explosion force of the dielectric fluid in a direction opposite to the machining subject 2, resulting in generation of vibrations. Such vibrations cause irregularities in the shape of the machining subject 2, resulting in degradation in precision.
Wire electric discharge machining is commonly employed in the process of manufacture of semiconductor devices. For example, the wire electric discharge machining is commonly employed in the case of the machining process for metal molds for IC lead frames. There have been demands for extremely high precision in the wire electric discharge machining and smooth surface roughness, such as 1 xcexcm in shape precision and not more than 1 xcexcmRmax in surface roughness, and an increase in applications requiring high productivity. In light of this demand, generation of vibration in the electrode is quite disadvantageous.
With respect to methods for solving such problems with the wire electric discharge machining in a solution, a technique relating to an air wire electric discharge machining that carries out a wire electric discharge machining process without interpolating a dielectric fluid between electrodes has been disclosed (xe2x80x9cImprovement of Accuracy of Second-Cut using Dry WEDMxe2x80x9d, Adachi, et al. Tokyo University of Agriculture and Technology, p154, Molding Technique, Vol. 14, 7th issue, 1999, Nikkan Kogyo Newspaper) This technique has disclosed that a wire electric discharge machining process in the air makes it possible to improve the precision in straightness in the machining subject cut surface. Therefore, the achievement is great from the viewpoint of providing high precision; however, the processing rate is as slow as approximately {fraction (1/10)} of the machining in a solution, resulting in a problem with practical use due to low productivity. Moreover, this technique has disclosed nothing about means for applications requiring higher precision or applications requiring high quality in the surface of the machining subject.
The present invention has been achieved to solve the above-mentioned problems. It is an object of this invention to provide a method of and apparatus for wire electric discharge machining that are suitable for high-precision machining with high productivity. Moreover, another objective is to provide a method of and apparatus for wire electric discharge machining that are suitable for high-quality machining.
The wire electric discharge machining method according to one aspect of this invention comprises generating an electric discharge in a gap between a wire electrode and a machining subject to thereby machine the machining subject, and supplying a pressurized gas in the gap between the wire electrode and the machining subject while the machining subject is being machined.
The wire electric discharge machining method according to another aspect of this invention comprises generating an electric discharge in a gap between a wire electrode and a machining subject to there by machine the machining subject, coarsely processing the machining subject in an environment of a dielectric fluid, finely processing the machining subject in a gaseous environment while supplying a pressurized gas in a gap between the wire electrode and the machining subject.
In the above-mentioned wire electric discharge machining method, the gas is at least one kind selected from the group consisting of oxygen, nitrogen, hydrogen, an inert gas and an insulating gas.
The wire electric discharge machining apparatus according to still another aspect of this invention comprises a discharge unit which generates an electric discharge in a gap between a wire electrode and a machining subject to thereby machine the machining subject, a gas supplying unit which supplies a pressurized gas in a gap between the wire electrode and the machining subject.
The wire electric discharge machining apparatus according to still another aspect of this invention comprises a discharge unit which generates an electric discharge in a gap between a wire electrode and a machining subject to thereby machine the machining subject, a dielectric fluid supplying unit which supplies a pressurized dielectric fluid in the gap between the wire electrode and the machining subject, and a gas supplying unit which supplies a pressurized gas in the gap between the wire electrode and the machining subject.
The above-mentioned wire electric discharge machining apparatus further comprises a switching unit which supplies a pressurized fluid into a nozzle and for switching this pressurized fluid to the dielectric fluid or the gas is provided so as to constitute the above-mentioned dielectric fluid supplying unit and the gas supplying unit.
In the above-mentioned wire electric discharge machining apparatus, the gas is at least one kind selected from the group consisting of oxygen, nitrogen, hydrogen, an inert gas and an insulating gas.